Lanthanum (La) chloride, a tracer smaller than colloidal La, is used here to follow the routes across cerebral vessels rendered premeable to ions and salts by hyperosmotic arabinose. Since La is invisible in the light microscope, horseradish peroxidase (HRP) is used together with the La. It has been correctly assumed that La has escaped within the region of visible HRP exudates. Serial sections have so far failed to reveal how the HRP crosses the endothelium, but the ionic La doses penetrate successive pools of extracellular space between tight junctions in arterioles. No complete penetration of these junctions, from vessel lumen to basal lamina has been found in single thin sections, however. Very few endothelial vesicles contain the La. The distribution of ionic La, the smallest tracer available, must be examined in many serial sections in order to follow its route of passage. In this way, possible differences in routes taken by protein and by salts or some ions may be uncovered.